Strain aging behavior can occur in almost all steels, including micro-alloyed steels used in high-strength pipelines. The direct effects of strain aging on mechanical properties can include increased hardness, yield strength and tensile strength, and reduced ductility and toughness. Strain aging may take place in processes where the pipe material experiences thermal cycles, such as coating, welding and in-service heating, and may occur with or without additional plastic strain. The changes of material mechanical properties could seriously challenge the design principles and methodologies, so that these aging effects need to be taken into account. This is especially important for pipelines expected to see deformation-controlled loading conditions. This is not only because the difference in strain aging effects between a weld and the parent material can easily change the strength overmatch condition of the weld, leading to unpredictable girth weld flaw tolerance, but also because the return of Lu¨ders behavior on the stress-strain curves of these materials significantly reduces the pipe buckling load resistance. In addition, any change in fracture resistance due to strain aging may impact the fracture control design practice, particularly if the pipe material may be expected to experience plastic deformation during service. In this paper, a brief review of strain aging behavior in steels is presented, with an emphasis on the effects on the mechanical properties and toughness of three high-strength line pipe steels. Material strain aging mechanical test procedures of three high grade pipes will be described and the test results will be discussed.
The ductile fracture toughness of steel is used to assess the ability of a pipeline to resist long running ductile fractures in a burst event. In modern low carbon clean steels with high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The major factor causing concern is the inability of these tests to isolate the energy associated with crack propagation from the total energy absorbed during the specimen fracture. This is significant in modern high toughness steels because their initiation toughness is extremely high. To resolve crack propagation energy, a novel modification was evaluated for both Charpy and DWTT specimens by employing a back-slot including a snug fitting shim to replace the removed material. In most cases, this modification was effective in curtailing the load-displacement trace when the propagating crack interacted with the slot on the backside of the specimen, without affecting the initial portion of the trace; this allowed crack propagation energies to be resolved. The propagation energy determined by this method is compared with the total energy and conventional test parameters. The crack propagation energy values inferred based on this should be validated, in future burst test.
As part of a safety case for a subsea 13Cr pipeline, the operator wished to demonstrate that if a circumferential through wall crack developed, the crack would remain stable as a leak rather than growing to a full bore rupture. An initial fracture mechanics analysis had suggested that the margins on crack length were too small to make such a “leak before break” argument. This paper reports an integrated programme of small scale testing, numerical modelling and full scale testing which showed that a leak before break case could be made. 13Cr martensitic steel generally shows excellent toughness at the service temperature, as does the super duplex weld metal that was used for the girth welds. However, as the pipeline had been installed by reeling, there was some concern that the toughness may have been reduced. Hence a programme of fracture toughness testing was designed to generate tearing resistance curves for both as-received and pre-strained parent material and weld metal. Deep and shallow through thickness notched specimen geometries were tested to explore the effect of constraint on the toughness. Finite element analysis was used to predict the stress intensity for a range of crack lengths, including the effects of misalignment. Non-linear analyses were used to estimate the limit load for the cracked pipe. The test results were used as input to tearing analyses to Level 3 of BS 7910. These showed that the tolerable length of a through wall crack exceeded the length of anticipated defects by a factor of at least two. To confirm the fracture mechanics predictions, two full scale tests were carried out. These used pressure cycling to grow a through wall crack by fatigue. These cracks were stable under an internal pressure equal to the pipeline design pressure. The cracked specimens were then axially loaded to failure. Extensive tearing occurred before final failure at loads above those predicted by the fracture analysis, confirming the conservatism of the predictions.
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